Siloxane paper release coatings



United States Patent 3,385,727 SILOXANE PAPER RELEASE COATINGS Peter S.Thomas, Livonia, Mich., and Thomas F. Maguire, Troy, N.Y., assignors toGeneral Electric Company, a corporation of New York No Drawing. FiledSept. 1, 1964, Ser. No. 393,738 Claims. (Cl. 117-155) ABSTRACT OF THEDISCLOSURE A paper release coating composition contains a silanolterminated gum, a silane containing 3 or 4 acetoxy groups, a metal saltand an organic solvent. A surface of a sheet of paper is coated with thecomposition, the solvent is removed and the silanol is crosslinked byreaction with the polyacetoxy silane in the presence of moisture at anelevated temperature. The coated paper produced is useful as a backingsheet for surgical adhesive tape.

This invention is concerned with rendering cellulosic materialsnon-adherent to various organic solids. More particularly, the inventionis concerned with a process for rendering paper or paperboardnon-adherent to normally adherent materials such as, for example,asphalts, bitumen, tars, waxes, paraffin solids, flour-containingpastes, and frozen foods, and other high molecular weight polymers andto the papers so treated. The process comprises treating the cellulosicmaterial with a mixture of ingredients comprising (1) a linearpolydimethylsiloxane containing terminal silicon-bonded hydroxyl groups,(2) a silane containing at least three acyloxy groups, and (3) anorganometallic salt.

Cellulosic fibers in the form of cellulosic papers and paperboard areused extensively as confining and shipping means for various highlyadhesive materials, including such organic compositions as asphalt orpitch, tar, various unvulcanized rubbers, particularly syntheticrubbers, other high molecular weight organic polymers used as adhesives,etc. For optimum use of the cellulosic containers, it is essential thatthey be capable of being readily separated or stripped from the cargowhich they contain or from other bodies which carry the highly adhesivematerials. Thus, in the transportation and shipment of asphalt used forroofing purposes, the asphalt is generally poured while still hot into acontainer, such as a carton, bag. or drum whose sides are cellulosic innature. After cooling, the asphalt becomes quite hard and can be readilytransported with little difiiculty. At the point of use, it is essentialthat the paper be capable of being readily stripped from the asphalt soas to permit easy access to the asphalt without adherence of extraneousportions of the paper or fibers which might undesirably affect theconstitution of the asphalt.

Various treatments have been accorded to these types of papers, oftenreferred to as anti-blocking paper or release paper. One method fortreating the paper to render it anti-blocking comprises treating the paer in a three-coating operation with (1) finely divided clay in casein,(2) finely divided clay, and (3) polyvinylacetate. Such paper isreleased by fracture of the clay coating, but the polyvinylacetateremains on the adhesive material. Another method commonly employed inthe art involves applying several thicknesses of polyethylene to thepaper, usually by treating the latter with solutions of thepolyethylene. A still further method for treating cellulosic material torender it non-adherent, particularly to asphalt, and to permit it to bereadily removed from direct contact with the latter, involves depositinga double coating on the cellulosic materials, the first 3,385,727Patented May 28, 1968 coating being of clay, and the second being ofmethylcellulose and starch.

The use of methylhydrogenpolysiloxanes in combination with water-solublecellulose ethers for the purpose of treating anti-blocking paper torender it less adherent to ordinary adhesive organic compositions haspreviously been described. Although such combinations of ingredients areoridinarily helpful in reducing the adhesive properties of the paper,much is left to be desired from such treatment of the paper. Often, therelease properties are unreliable and the release characteristics arenot uniform throughout the surface of the paper. In addition, it isessential, in order to obtain optimum release properties, that thetreated paper be aged for extended periods of time, e.g., by storage,before it is useful for release purposes. This storage is necessarybecause accelerated aging by high-temperature treatment is not usuallyavailable commercially in paper-treating establishments. Furthermore,paper such as parchment paper, cannot be heated above C. withoutdeleteriously affecting the paper. Moreover, after treatment of thepaper with methylhydrogenpolysiloxane, the abhesive characteristics(i.e., the release properties) tend to decrease with time so that afterlong-term storage, such treated paper no longer shows abhesivecharacteristics.

In addition to the diificulties described above, particularly when usingmethylpolysiloxanes for anti-adhesion (abhesive) purposes, and even whenusing the more currently employed methylhydrogenpolysiloxanes for thispurpose, it has been found that, although release properties areimproved, nevertheless, there is an undesirable tendency of the siliconein the abhesive paper to migrate to the surface of the paper, therebycoming in contact with the material which it is desired to release.Often the abhesive paper is in contact with the compostions which aredestined to be used for adhesion applications, and the tack or adhesionis undesirably reduced as a result of this migration of theorganopolysiloxane from the treated release paper to the adhesive.

An additional problem is presented when cellulosic fiber sheets arecoated with polysiloxanes for the purpose of forming release paper to beused in conjuncti n with adhesives tapes and labels. Many of thecoatings of the prior art have a tendency to migrate from the treatedsurface to an adjacent uncoated side, making that surface unprintable.Additionally, such coatings require a high temperature for cure, e.g.,250 F.-300 PI, and if cured at temperatures below 250 F., may not onlymigrate, but can cause blocking or sticking together of the adjacentlayers When the coated paper is rolled up. Migration also causes a lossof adhesion in pressure-sensitive adhesive tapes and labels which comeinto contact with the coated paper. One method employed to achieve thedesirable properties without high temperature curing was the use ofmethyltrichlorosilane [(CH )SiCl as a low temperature curing agent forthe polysiloxanes in the treating solution. However, the by-product,hydrogen chloride, generated by the curing reaction using themethyltrichlorosilane contaminated and corroded the coating equipment sothat the practice of using this material as a curing agent wasabandoned.

In accordance with our invention, a method has been found for providinga non-blocking siloxane release coating on a cellulosic fiber sheetwhich can be cured at low temperature to produce a non-migratingcoating, while still providing the desired release properties.

It is, therefore, one object of this invention to provide a method forrendering cellulosic fiber sheet materials non-adherent to normallyadhering substances by treating with a coating material which can becured at low temperatures.

3 It is a further object of this invention to provide a method forrendering cellulosic fibrous sheet materials non-adherent to normallyadhering substances by treatment with a solution containing a lowtemperature curing, non-migrating, non-smearing polysiloxanecomposition.

It is a still further object to produce a cellulosic fibrous sheetmaterial treated in accordance with this invention. Briefly, thecompositions which are used to obviate the above-mentioned problems andprovide a release coating which is cured at low temperatures comprisesolvent solutions of (1) a linear polydimethylsiloxane gum havingterminal silicon-bonded hydroxyl groups, (2) a silane having at leastthree acyloxy substituents, e.g., methyltriacetoxysilane, and (3) anorganometallic salt, such as dibutyl tin dilaurate.

The polydimethylsiloxanes employed in the practice of the presentinvention are those having the general formula,

where n is an integer equal to at least 4500, for example, 4500 to 7500.These polydimethylsiloxanes, containing terminal silicon-bonded hydroxylgroups are soluble in organic solvents such as benzene, toluene, xylene,trichloroethylene, etc. The minimum average number of dirnethylsiloxyunits is required to provide a penetration level for the gum sufiicientto give the necessary properties to the ultimate release coating. Themaximum penetration which can be used is about 1000, a penetrationcorresponding to a dimethylpolysiloxane gum having a molecular weight ofabout 350,000 and a viscosity of about 10,000,000 centistokes. Thepeneration is measured in 0.1 mrn/min. using A.S.T.M. Standard Test No.D-217-60 T with a modified plunger or foot. The plunger or foot utilizedto measure the penetration of the organopOlysiloxane gums described inthis application consists of a cylinder A in diameter and A long formedof brass and attached to a shaft of steel having a diameter of A2 and alength of approximately This plunger Weighs approximately 9.1 g. Forpurposes of the test, a 100 g. load is placed on the shaft.Correspondingly, a d methylpolysiloxane gum with a molecular weight ofabout 550,000 has a viscosity in the range of 50,000,000 centistokes anda penetration of approximately 100. The lower penetration gums providecoatings which are smear-resistant, and thus are the most desirable forpurposes of release. The preferred gums are those having the lowestpenetrations, and thus the highest molecular weights.

These polydimethylsiloxanes can be prepared by any one of several wellknown methods. Thus, the high viscosity polydimethylsiloxanes can beobtained by condensing the hydrolysis product of dimethyldichlorosilanewith either acidic or alkaline catalysts such as hydrochloric acid,sulfuric acid, potassium hydroxide, etc. Alternatively, one can heatcyclic polymers of the formula where m is an integer equal to from 3 to6, for instance, octamethylcyclotetrasiloxane, with an alkaline catalystsuch as potassium hydroxide in an amount of from 0.001 to 0.1% based onthe weight of the octamethylcyclotetrasiloxane, at temperatures of from125 C. to 175 C. for times ranging from about 15 minutes to 2 hours ormore and thereafter, if desired, removing the alkaline catalyst. Bytreating the material with water and heating, terminal hydroxyl groupswill be formed on the polydimethylsiloxane. This yields apolydimethylsiloxane of Formula 1 having a viscosity of from about10,000,000 to 50,000,000 or more centipoises when measured at 25 C.

The organosilicon compound which is used to cure the silanolchain-stopped dirnethylpolysiloxane gum is selected from compoundshaving the formula,

where R is a monovalent hydrocarbon radical free of aliphaticunsaturation, R is an alkyl group having from 1 to 4 carbon atoms, and ais an integer from 0 to 1. More specifically, the R substituent can bean alkyl group such as methyl, ethyl, propyl, etc.; a cycloalkyl groupsuch as cyclopentyl, cyclohexyl, etc.; an aryl group such as phenyl,naphthyl, biphenyl, tolyl, xylyl, ethylphenyl, etc.; an aralkyl groupsuch as benzyl, phenylethyl, etc. Additionally, R can be a monovalenthydrocarbon radical substituted with groups such as cyano, carbalkoxy,and halogen substituents, e.g., beta-cyanoethyl, gamma-cyanopropyl,betacarhoxyethyl, beta-carbethoxyethyl, gamma-hydroxypropyl,chloromethyl, dibrornophenyl, etc. The R substituent can be an alkylgroup having from 1 to 4 carbon atoms such as methyl, ethyl, propyl,isopropyl, butyl, isobutyl, and t-butyl.

In addition to the acyloxy substituted silanes described above, cyclicsiloxane compounds such as those having the formula, [(R'COO)(CH )SiO]where R is as de scribed above and b is an integer from 3 to 6, can beused. Further acyloxy substituted silmethylene compounds such as thosehaving the formula,

where R is as defined above, will cure the silanol-terminateddimethy'lpolysiloxane gum. However, the results obtained with theacyloxy-substituted silmethylenes are less satisfactory than thoseobtained using either the trior tetra-substituted silane or theacyloxy-substituted cyclic siloxane. The restriction that the Rsubstituent be an alkyl group of less than four carbon atoms isnecessary because the acids formed as a by-product in the reaction arenot easily removed from the reaction mixture if they contain more thanfive carbon atoms.

An excess of the acyloxy-substituted silane, with relation to thesilanol chain-stopped gum, is always necessary. To provide this excess,the ratio of acyloxy substituted silane having the Formula 2 to thedimethylpolysiloxane gum containing silanol terminals should be in therange of from 0.5 to 1.5 weight percent of the silane based on thesilanol chain-stopped gurn. The preferred range of silane is from 0.75to 1.25 weight percent. If less than 0.5 weight percent of theacyloxy-su'bstituted silane is present, the result is an insufficientlycured material. If the level of the acyloxy-substituted silane is raisedbeyond 1.5%, the finally cured coating does not possess the desiredrelease properties.

Various organometallic salts can be used as catalysts with the silanolgum-acyloxy substituted silane mixture. In general, these catalystscomprise the organometal salts of fatty acids. Exemplary of such saltsare dibutyl tin dilaurate, zinc octoate, stannous octoate, dibutyl tindioctoate, etc. Based on the silanol chain-stopped gum, there should befrom 0.3% to 1.2%, as metal, of the catalyst. Below the level of 0.3%catalyst, sufiiciently rapid cure at low temperatures is not achieved;if more than 1.2% of the cure-prornoting catalyst is present, anundesirable film of the salt remains on the surface of the polysiloxanecoating.

The polysiloxane coating is applied to the paper through the use of asolvent solution of the various materials. Stability of the solution,with respect to shelf life, depends upon the concentration oforganosilicon materials in the solution and whether both the silanolchain-stopped dimethylpolysiloxane gum and the acyloxy-substitutedsilane are present in the solution during storage. When both the gum andthe acyloxy-substituted silane are present in the same solution, themaximum concentration to provide a reasonable shelf life stability isapproximately 10% solids. With 10% or less solids, a shelf life in therange of three months may be achieved. It is uneconomical to use thesolution below a concentration of approximately 1%. If two solventsolutions are provided, i.e., one solvent solution containing thesilanol chain-stopped dimethylpolysiloxane gum and one solutioncontaining the acyloxy-substituted silane, no shelf life problems arepresented even when the concentrations of each of the materials in itsrespective solution is 30%. Here, obviously, the two solutions are mixedat the point of use so that shelf life is no longer a problem.

Thus, within the limits stated above, the coating solutions of thepresent invention can have an overall composition, by weight, asfollows:

Parts (1) Dimethylpolysiloxane gum containing silanol chain terminals1-30 (2) Acyloxy-substituted silane 0005-045 (3) Organometal salts offatty acids 1 0.003-036 (4) Solvent 70-99 1 As metal.

A preferred range of substituents for the composition is as follows:

Parts (1) Dimethylpolysiloxane gum containing silanol chain terminals1-30 (2) Acyloxy-su'bstituted silane 0008-0375 (3) Organometal salts offatty acids 1 0003-036 (4) Solvent 70-99 1 As metal.

A wide variety of solvents can be used for the coating solution.Particularly, the solvents can be aromatics such as benzene, toluene,and xylene. Additionally, solvents such as tetrahydrofuran, esters suchas ethyl acetate, ketones such as methyl ethyl ketone, and substitutedaromatic compounds such as toluene and chlorobenzene may be utilized.Solvents except for those just mentioned, while they may dissolve thevarious reactants, present shelf life problems in addition to thosementioned with relation to solution concentration. The order of additionof the various reactants to the solvent solution is immaterial.

As was previously described, the purpose of the present invention is toprovide a solution which allows a release coating to be placed on paperwithout the necessity for a high temperature cure. Moisture, as from theair, will initiate curing of the material of the present invention, evenat room temperature, but about 20 hours is required to complete the curein the absence of a catalyst.

When the cure-promoting catalyst is present in the solution, attemperatures of from 150 to 200 F., the various coatings may be cured inapproximately 30 seconds. If the reaction temperature is lowered to 100F., the cure will require approximately 1 minute. When the paper iscoated in high speed paper coating machinery, a cure time of about 1minute is the longest which may be tolerated, so that 100 F. is thelowest practical curing temperature. However, of course, if high speedpaper coating equipment is not used, and it is only desired to provide arelease coating on paper using a material which may be cured at a lowtemperature, the material of the present invention can easily be usedwith curing temperatures below 100 F. or without a catalyst.

The coating of the present invention is normally used on glassine paperor on clay coated paper. For practical release applications, thereshould be a uniform coating of the cured dimethylpolysiloxane gum in anamount of approximately /2 pound per 3,000 square feet. Tomake sure thatthe entire paper is coated, up to 1 pound of dimethylpolysiloxane gumcan be employed per 3000 square feet; greater amounts of gum beinguneconomical. A release coating which is workable, however, is producedwith 0.2 pound dimethylpolysiloxane gum per 3000 square feet providedthere is uniform coverage.

Following treatment of the cellulosic material with the solution of thepresent invention, the cellulosic material is advantageously dried bypassing the treated paper over heated rolls maintained at temperaturesof about 100 to 200 F. for up to one minute. The use of circulating hotair at similar temperatures may also be used for times of from 15seconds to one minute to effect curing of the treated paper. This dryingstep will bring out the optimum release properties of the paper withoutfurther heat treatment, and such optimum release properties areimmediately available without requiring aging or storage of the treatedpaper.

The following examples are illustrative of the use of the process of thepresent invention and should not be considered as limiting in any wayits full scope, as covered by the appended claims.

Example 1 A treating bath was prepared containing a 10% solution, intoluene, of a silanol chain-terminated dimethylpolysiloxane gum,methyltriacetoxy silane in the amount of 1% based on thedimethylpolysiloxane gum and a solution of dibutyl tin dilaurate andxylene containing 6% tin, in an amount of 10% based on thedimethylpolysiloxane gum. The polysiloxane utilized for this treatingbath had the same properties as the gum described in Example 1. The bathwas applied to a claycoated paper by standard metering techniques toprovide a coating of one pound of gum per 3000 square feet of paper. Thepaper was then exposed to the air at room temperature and subsequentlyheated for 1 minute at F. The paper-coated as just described was foundto have release properties normally found in coatings cured at highertemperatures.

The release-coated paper was tested using Johnson and Johnson SurgicalTape and found to have an adhesion of 5-10 ./in. width. There was noevidence of migration when the tapes were subjected to the subsequentadhesion tests prescribed by the proposed TA'PPI procedure. The testinvolves placing the tape which has been removed from theorganopolysiloxane-coated paper on a clean 302 stainless steel panelhaving a 12 to 16 microinch ground finish. The tape is rolled with a 4.5pound roller once in each direction and is then allowed to remain for 30minutes at 73 F. and 50% relative humidity. The migration is determinedby pulling the tape from the stainless steel panel and comparing theadhesion with that of tape which has not previously been applied to anorganopolysiloxane-coated paper. The adhesion obtained for the paper wasthe same when it was tested immediately after curing, after aging for 1month, and after aging for 2 months.

CONTROL EXAMPLE A 10% solution of a silanol-stopped dimethylpolysiloxanegum, in toluene, and 1%, based on the siloxane of methyltriacetoxysilane[(CH )Si(OOCCH were mixed together. The polysiloxane had a molecularweight of about 500,000, a viscosity of 35x10 centistokes, and apenetration of 150. A glassine paper was coated with the mixture usingstandard metering techniques to provide one pound of gum per 3000 squarefeet of paper. The paper sheet was exposed to air at room temperaturefor a period of about 20 hours. Approximately 10 minutes afterapplication of the polysiloxane solution, the paper was tested and foundto be non-blocking. However, release results were not optimum in thatexcessive migration was observed. There was a slight loss of tapeadhesion indicating that, at this point, the coatin was not completelycured. A portion of the paper was again tested approximately 20 hoursafter application of the treating solution. Following this longerperiod, the dimethylpolysiloxane coating was found to be completelycured.

Example 2 A coating bath was prepared by mixing a 10% solution, intoluene, of a silanol chain-terminated dimethylpolysiloxane gum, andmethyltriacetoxysilane in an amount of 1% based on thedimethylpolysiloxane gum, and a solution of dibutyl tin dilaurate inxylene containing 6% tin, in an amount of 10% based on the gum. Thedimethylpolysiloxane gum had a molecular weight of approximately425,000, a viscosity of about 18x10 centistokes, and a penetration of675. Glassine paper was coated utilizing standard metering techniques toprovide a coating of one pound of gum per 3000 square feet of paper. Thepaper was exposed to the air at room temperature and then cured for 1minute at 150 F. Immediately after completion of the cure, the paper wastested for release using Johnson and Johnson Surgical Tape and theadhesion was observed to be -10 g./in. width. Following 2 months aging,the adhesion had not changed substantially.

It is apparent from this disclosure that a practical method of treatingcellulosic fibrous sheets at substantially reduced temperatures,including room temperature, to provide a release coating, which isnon-blocking and non-migratory, has been provided. The amount ofsilanolterminated dimethylpolysiloxane, acyloxy-substituted silane, andorganometal salt catalyst can be varied within wide limits as previouslydisclosed. The materials and process described are equally applicable toone-step coating processes and to continuous processes for providingrelease coatings on cellulosic fibrous materials. Standard paper makingor paper converting equipment can be readily employed in connection withthe treating operations and no precautions need be taken for any toxicmaterials which may be contained in the solutions, other than thosenormally observed for the particular organic solvents.

The cellulosic materials treated as described above, have a wide rangeof usefulness. Thus, asphalt or high molecular weight organic polymers,such as various synthetic rubbers, can be poured hot into any containersfashioned from the treated paper or paperboard, and after cooling itwill be found that the solidified asphalt or polymer is readily andcleanly separated from the container wall.

Further, the invention permits paper treated in accordance with thisprocess to be substituted for various fabrics which heretofore have beenused in contact with the adhesive surfaces of materials such aselectricians pressure-sensitive tape, adhesive tape used for surgicalpurposes, and regenerated cellulose tape carrying a permanent adhesiveupon one surface. Vulcanized or unvulcanized sheets or rubber can beprevented from adhering despite the fact that these sheets of rubber maybe quite sticky and cohesive when in direct contact with each other.Paper treated in accordance with the instant invention is also useful inlining various boxes of partially prebaked goods such as buns, rolls,and the like, and advantage can be taken of the outstanding releaseproperties at elevated temperatures by completing the baking cycle inthe original container in which the baked goods are purchased.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A method for rendering cellulosic fibrous sheet material non-adherentto surfaces which normally adhere thereto, comprising treating the sheetmaterial with a bath consisting essentially of, by weight, the followingingredients:

(1) from 1 to 30 parts of a linear dimethylpolysiloxane gum containingterminal silicon-bonded hydroxyl groups and having a viscosity of atleast 10,000,000 centistokes at 25 C.,

(2) from 0.005 to 0.45 part of a silane having the formula RaSi(OOCR')where R is a monovalent hydrocarbon radical free of aliphaticunsaturation, R is a monovalent alkyl radical having from 1 to 4 carbonatoms, and a is 1,

(3) from 0.003 to 0.36 part, as metal, of an organometal salt, and

(4) from to 99 parts of an organic solvent, and thereafter heating thecellulosic fibrous base to at least 100 F. to effect a cure and removethe solvent.

2. The method of claim 1 wherein the silane is methyltriacetoxysilane.

3. The method of claim 1 wherein the silane is present in an amount offrom 0.008 to 0.375 part.

4. The method of claim 1 wherein the organometallic salt is dibutyl tindilaurate.

5. A method for rendering cellulosic fibrous sheet material non-adherentto surfaces which normally adhere thereto, comprising a treating thesheet material with a bath consisting essentially of, by weight, thefollowing ingredients:

(1) from 1 to 30 parts of a linear polydimethylsiloxane gum containingat least one terminal silicon-bonded hydroxyl group and having aviscosity of at least 10,000,000 centistokes at 25 C.,

(2) from 0.005 to 0.45 part methyltriacetoxysilane,

(3) from 0.003 to 0.36 part, as tin, dibutyl tin dilaurate,

and,

(4) from 70 to 99 parts toluene,

and thereafter heating the cellulosic fibrous base to at least 100 F. toeffect a cure and remove the solvent.

References Cited UNITED STATES PATENTS 2,985,544 5/1961 Monterey et a1.ll7l61 X 2,985,545 5/1961 Leavitt 1l7161 X 2,985,546 5/1961 Leavitt1l716l X 3,035,016 5/1962 Brunet 117-161 X 3,133,891 5/1964 Ceyzeriat117-161 X WILLIAM D. MARTIN, Primary Examiner.

